Vertical-cavity surface-emitting devices generally use an index guide to provide mode confinement, and this index guide can also provide electrical confinement. The oxide-aperture vertical-cavity surface-emitting laser (VCSEL) is one such device for which the oxide acts as an electrical insulator to provide electrical confinement, while the oxide aperture also produces a mode-confining region that provides transverse mode confinement. This improves the efficiency and modulation speed of the VCSEL.
While the oxide aperture has been successfully used in many VCSEL devices, it has drawbacks for device operation as well as manufacturing and reliability. The material differences between the oxide aperture and remaining semiconductor can cause high internal mechanical strain. Because of this strain and chemical reactivity in the atmosphere, the exposed sidewalls for the semiconductor post etched to create the aperture can cause slow delamination of semiconductor mirror layers and early device failure. In addition the oxide material formed in the semiconductor mirror creates an effective heat block, and increases thermal resistance. The oxide generally has a different thermal expansion coefficient than the oxide, and proceeds through a timed diffusion process that results in aperture size variation across a processed VCSEL wafer.
Other techniques such as buried tunnel junctions covered with conducting epitaxy in a subsequent epitaxial regrowth have also been used to provide mode confinement and electrical confinement in VCSELs. However these approaches can also suffer from poor quality of the epitaxial material due to regrowth if the materials include Al, and can leave a relatively large surface epitaxial step height for which regrowth should be performed over. The problems with epitaxial regrowth are particularly difficult because of the reliance on Al-bearing materials (e.g., AlGaAs or AlGaN) for high quality mirrors. Because of these difficulties however, these types of vertical-cavity surface-emitters have had difficulty in matching the light vs. current and voltage vs. current characteristics that have been possible using oxide apertures. Surface impurities that become interface impurities upon regrowth can be particularly problematic. The surface impurities can also cause poor nucleation during regrowth and surface roughening in the epitaxially regrown layers.
Al-bearing materials generally oxidize according to their Al content. AlAs for example, rapidly oxidizes when exposed to air and can disintegrate and/or fully become detached from the remaining epitaxy and substrate. AlGaAs materials oxidize with a rate that depends on Al-content. The poor regrowth interface due to oxygen and other contaminants such as carbon can then damage or degrade key regions of the VCSEL when regrowth is attempted directly on AlGaAs. Although GaAs has a relatively low oxidation rate in air and has been used as a layer for regrowth of epitaxial mirrors, it is also not an ideal material on which to perform regrowth for many vertical cavity light emitters. The GaAs material itself will also oxidize when exposed to air, and furthermore can cause absorption problems in the mirror of the device for wavelengths shorter than ˜880 nm at room temperature.
Therefore improvement in the performance of the vertical cavity light source can be obtained by eliminating the need for a material for GaAs on which to perform regrowth in active area of the light source. The same is true for vertical cavity light sources made in other material systems such as based on nitrides. As a result, despite efforts in developing high quality and fully epitaxial structures for Al-bearing vertical-cavity surface-emitters that can match or exceed the electro-optic performance of the oxide VCSEL while solving problems due to the oxide aperture, these VCSELs have so far in general remained inferior in their device operation and other features to oxide-apertured VCSELs due to either poor interface and/or material quality, or undesirable materials that cause absorption in the vertical cavity light source. Because of oxide aperture technology has thus remained dominant, and the Al-bearing vertical-cavity devices have not yet reached their full potential in electro-optic performance. The technology of VCSELs therefore has a remaining need for vertical-cavity device that can provide epitaxial mode confinement using high quality epitaxial regrowth, while being able to engineer the epitaxial mode confinement for high electro-optic performance.